Before I updated my OBS settings to record to .mp4 files I manually converted .mkv files to .mp4. This shell command does that for every recording in a directory deleting the original.

$ for x in $(ls *.mkv | awk -F "." '{print $1}') ; do ffmpeg -i $x.mkv -c copy $x.mp4 && rm $x.mkv ; sleep 3; done